An acoustic charge transport device ("ACT device") is a semiconductor device utilizing improved surface acoustic wave principles for transporting charge in a buried channel between an input and an output. An exemplary ACT device is disclosed in U.S. Pat. No. 4,633,285, issued to B. Hunsinger, et al for ACOUSTIC CHARGE TRANSPORT DEVICE AND METHOD, the disclosure of which is incorporated herein by reference.
The ACT device is a high performance circuit element which is fabricated on a thick, lightly doped gallium arsenide epitaxial substrate. The ACT device suffers from large insertion loss caused by high terminal impedance which complicates the difficulty of integrating ACT devices with other signal processing components.
A field-effect transistor ("FET") is a circuit component in which the resistance of the current path between the source and the drain is modulated by applying a transverse electric field through utilization of a gate. A metal semiconductor field-effect transistor ("MESFET") is normally fabricated through ion implantation of a p-type layer having doping density and layer thickness between the thin, highly doped n-type implanted layer used for the active MESFET channel, and the thick, lightly doped n-type epitaxial layer used for charge transport in an ACT device.
As noted, the ACT device and MESFET device are customarily fabricated with differing channel thicknesses and doping densities, and integration of them into a usable circuit element therefore presents a number of complicating factors.
The disclosed invention is a circuit element comprised of an ACT device which is operably connected with a MESFET device in a manner which overcomes the high insertion loss of the ACT device, thereby permitting additional signal processing components to be configured that extend the application areas for ACT device technology. MESFET performance is improved by the design of the invention and may be integrated directly onto the thick epitaxial layer. The incorporation of a p-type well implant provides improved MESFET reproducibility, particularly when compared to conventional MESFET devices fabricated through ion implantation into semi-insulating substrates.